Under-Keel Clearance Calculator
Under-keel clearance is the water you have left between the bottom of the hull and the seabed. To find it, start from the charted depth, add the predicted height of tide to get the actual water depth, then subtract the vessel's draft and the squat caused by moving through shallow water. A safety margin on top absorbs the uncertainty in charts, tides, and density. Enter charted depth, tide height, draft, squat, and your safety margin, and this calculator returns gross and net under-keel clearance with a go or no-go verdict.
Under-keel clearance formula
Available depth = charted depth + height of tide
Gross UKC = available depth - draft - squat
Net UKC = gross UKC - safety margin
Net UKC >= 0: safe water | Net UKC < 0: insufficient
Charted depth is relative to chart datum, so the height of tide above datum is added. Squat is the dynamic sinkage from movement. The safety margin covers residual uncertainty in all inputs.
Clearance notes
- Use tide height for the actual time of transit, not high or low water.
- Squat grows roughly with the square of speed; slowing down restores clearance.
- Many ports set a minimum UKC, often a percentage of draft; honour the stricter figure.
- Allow extra for swell, density less than charted, and survey age.
- Re-check clearance at the shallowest point of the planned track.
Under-keel clearance: frequently asked questions
What is under-keel clearance?
Under-keel clearance (UKC) is the vertical distance between the lowest point of a vessel's hull and the seabed. It equals the available water depth minus the vessel's static draft and dynamic effects like squat. A positive UKC means the vessel is afloat with water to spare; zero or negative means grounding.
How is under-keel clearance calculated?
Available depth equals charted depth plus the height of tide above chart datum. UKC equals available depth minus static draft minus squat. So UKC = (charted depth + tide height) - draft - squat. A safety margin is often subtracted on top of this.
What is squat?
Squat is the bodily sinkage and trim a vessel experiences when moving through shallow water, caused by reduced pressure under the hull. It increases with speed and in confined water, and can be a significant fraction of a meter for large ships. Enter your estimated squat for the speed and depth.
Why add a safety margin to under-keel clearance?
Charted depths, tidal predictions, density, swell, and squat estimates all carry uncertainty. A safety margin, often a percentage of draft or a fixed allowance set by the port, absorbs that uncertainty so the vessel never touches bottom. Many ports mandate a minimum UKC for transit.
Where do I find charted depth and tide height?
Charted depth is read from the nautical chart relative to chart datum. Tide height for the time of transit comes from NOAA tide predictions for the nearest reference or subordinate station. Both are location-specific, so they are user inputs here.
Official sources
- NOAA Tides and Currents: Tides and Currents.
- NOAA Office of Coast Survey: Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.